Other resources for more information on HART and Foundation Fieldbus (FF)
FieldComm Group - Organization that owns and manages the HART protocol specification.
Romilly's - Consultant with links and other useful information for the HART user.
Control Forum - Forum discussion regarding the 250 ohm HART loop resistance.
HART Communication Protocol: A Practical Guide - Book available on Amazon
HART Technology: A Technical Overview - Book available on Amazon